1. Field of the Invention
The present invention relates to a signal superimposing apparatus and a signal superimposing method which use a power signal of an alternate current (AC) power line as a carrier wave for use in a power line communication for transmitting an information signal, and in particular to a telecommunication apparatus and a telecommunication method which enable a telecommunication between a base station and a terminal station through a power line forming a many-to-many multiple access network of a code division multiple access type.
2. Description of the Related Art
Today, a power line communication (PLC) is considered to be effective as one of information transmission means for building up a ubiquitous society, which requires a very large transmission capacity and speed. In spite of the fact that networks, such as the Internet, utilizing indoor power line have actually been built up in Europe and America, little progress has been made in a development of a multiple channel code division multiple access type enabling a high speed simultaneous access of a plurality of terminals. Meanwhile, conceived as a multiple access system includes a method for dividing media into a plurality of channels and allocating them to individual stations, a method for each station measuring a usage condition of telecommunication media and performing a transmission control autonomously (i.e., a random access) and a multiple access system other than the random access; a commonly used method, however, is a random access system.
This method has only been capable of a time division telecommunication. For example, although a method of transmitting packetized data after confirming that other equipment performing an information and telecommunication on a power line, such as a Carrier Sense Multiple Access (CSMA) is adopted by many standards and products, a simultaneous access has not been enabled. As for other methods other than a random access, the considered as a multiple access system includes a frequency division multiple access (FDMA), a time division multiple access (TDMA); the frequency division multiple access (FDMA) type, however, is not suitable to a power line telecommunication in which a characteristic is widely different depending on a frequency. As for the TDMA type, it has been conventionally believed to be very difficult to accomplish it in a power line environment because the TDMA requires a common clock among each station.
And conventionally known is that a code division multiple access (CDMA) type is a method suitable to a power line telecommunication system as a method in which each station uses the same frequency by a spectrum diffusion technique and which does not require a central control. Also, the code division multiple access type, which superimposes a diffusion signal in a frequency dimension, has been believed to be desirable in terms of consistency with a modulation system, in place of a direct diffusion code division multiple access type which superimposes a diffusion signal in a time dimension. However, it has been difficult to accomplish a many-to-many simultaneous access in a power line telecommunication by the CDMA.
Meanwhile, a conventional power line communication method uses a sine wave power signal as carrier wave and sends out an information signal by superimposing with the power signal. Then, a receiving side separates the information signal superimposed with the power signal by using a filter. In this case, if an information signal is separated by a characteristic of a filter when the information signal is separated from the power signal, a phase of the information signal is shifted by the filter characteristic, which has caused no fundamental problem for the information signal being an analog signal. And if a digital signal is transmitted by the above described power line communication, a phase of the digital signal is shifted on the receiving side, resulting in deforming a waveform on the time axis.
In the case of transmitting a pulse signal by a pulse code modulation (PCM) system, however, the receiving side is capable of generating a pulse signal anew thus being enabled to respond to a distortion if the waveform is distorted on the time axis.
However, in the case of multiplexing a signal by applying a spectrum diffusion system, the receiving side is not capable of generating a pulse signal anew, as in the case of the PCM system. The distortion of the waveform on the time axis at the time of receiving a signal is actually a large issue of the spectrum diffusion system and CDMA.
In addition, another known method as one of the conventional power line communication systems is one for superimposing an information signal by making a sine wave power signal of an AC power line as a carrier wave. It is used, for example, for transmitting a relatively simple control signal such as an on/off control of a motor and an analog signal such as voice, and transmitting a digital signal by the PCM system. And most of the signal superimposing apparatuses used for the system is configured to superimpose an information signal on a power signal by making a connection transformer intervene between itself and the AC power line as a transmission path. There is accordingly a known fact that a phase, et cetera, of the information signal is changed by a certain filter which is formed by the connection transformer and AC power line, thus giving a waveform distortion to the superimposed information signal.
Even if such a waveform distortion is generated in an information signal, however, it has not caused a fundamental obstacle to the above noted control signal or analog signal because of the characteristic of a signal process, nor has it caused a difficulty to a digital signal used in the PCM system, to the extent of being incapable of repairing a code error due to a waveform distortion by means of a signal reproduction process on the reception side, and therefore a system level problem has not occurred.
Meanwhile, in the midst of various multiple access systems being considered in association with a progress of multiple access techniques in recent years, especially attracting attentions is the CDMA employing the spectrum diffusion multiplex system enabling a multiplexing by a simpler comprisal as compared to the frequency division multiple access system and time division multiple access system. However, if an information signal multiplexed according to the CDMA is attempted to transmit by the above noted power line communication system, a problem of an inability of demodulating the information signal arises because an inverse diffusion on the reception side becomes difficult due to a waveform distortion of the information signal. That is, the power line communication system using a power signal of an AC power line as carrier wave and superimposing an information signal on the power signal is incapable of superimposing the information signal of a desired waveform without a waveform distortion, and therefore a problem of being unable to transmit the information signal by the CDMA occurs.
In the meantime, patent documents 1, 2 and 3 have disclosed a telecommunication apparatus and method enabling a transmission by the CDMA by using a power line allowing a superimposition of an information signal of a desired waveform on a power signal of an AC power line. Particularly, provided are a telecommunication apparatus and method using a power line allowing superimposing an information signal of a desired waveform on a power signal of an AC power line (e.g., single phase, three-phase) having a ground or neutral point.
These telecommunication apparatus and method comprise a system (i.e., one-to-many communication) transmitting a code multiple division multiple point access-use information signal for simultaneously carrying a plurality of different pieces of information (e.g., an image, voice, facsimile, TCP/IP Protocol) from one service outlet, to many ones by transmitting a code multiple division multiple point access-use information signal by a voltage source from each service outlet and therefore other service outlets are only capable of receiving during the time when either service outlet is used for a transmission, hence precluding a use of a system (i.e., many-to-one communication) which simultaneously transmits a code division multiple point access-use information signal for simultaneously carrying a plurality of different pieces of information independently from different service outlets and receives at either one thereof, or a many-to-many simultaneous access of a code division multiple access type power line communication.
Furthermore, the above noted patent documents do not provide either (1) how to synchronize each code signal or (2) a method for forming a code multiplex signal both of which are the most important problems in accomplishing the CDMA for general uses including a portable phone in order to form a code multiplex signal from a synchronous signal on a power line and independent signals from individual service outlets and superimpose the code multiplex signal on the power line.
[Patent document 1] Registered U.S. Pat. No. 6,194,996
[Patent document 2] Laid-Open Japanese Patent Application Publication No. 11-88239
[Patent document 3] Laid-Open Japanese Patent Application Publication No. 2002-111554